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Re: [PATCH] MIPS: Make BUG() __noreturn.

To: David Daney <ddaney@caviumnetworks.com>
Subject: Re: [PATCH] MIPS: Make BUG() __noreturn.
From: Andrew Morton <akpm@linux-foundation.org>
Date: Fri, 21 Nov 2008 15:00:23 -0800
Cc: linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
In-reply-to: <49260E4C.8080500@caviumnetworks.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <49260E4C.8080500@caviumnetworks.com>
Sender: linux-mips-bounce@linux-mips.org
On Thu, 20 Nov 2008 17:26:36 -0800
David Daney <ddaney@caviumnetworks.com> wrote:

> MIPS: Make BUG() __noreturn.
> 
> Often we do things like put BUG() in the default clause of a case
> statement.  Since it was not declared __noreturn, this could sometimes
> lead to bogus compiler warnings that variables were used
> uninitialized.
> 
> There is a small problem in that we have to put a magic while(1); loop to
> fool GCC into really thinking it is noreturn.  This makes the new
> BUG() function 3 instructions long instead of just 1, but I think it
> is worth it as it is now unnecessary to do extra work to silence the
> 'used uninitialized' warnings.
> 
> I also re-wrote BUG_ON so that if it is given a constant condition, it
> just does BUG() instead of loading a constant value in to a register
> and testing it.
> 

Yup, this change will fix some compile warnings which will never be
fixed in any other way for mips.

> +static inline void __noreturn BUG(void)
> +{
> +     __asm__ __volatile__("break %0" : : "i" (BRK_BUG));
> +     /* Fool GCC into thinking the function doesn't return. */
> +     while (1)
> +             ;
> +}

This kind of sucks, doesn't it?  It adds instructions into the kernel
text, very frequently on fast paths.  Those instructions are never
executed, and we're blowing away i-cache just to quash compiler
warnings.

For example, this:

--- a/arch/x86/include/asm/bug.h~a
+++ a/arch/x86/include/asm/bug.h
@@ -22,14 +22,12 @@ do {                                                        
        \
                     ".popsection"                              \
                     : : "i" (__FILE__), "i" (__LINE__),        \
                     "i" (sizeof(struct bug_entry)));           \
-       for (;;) ;                                              \
 } while (0)
 
 #else
 #define BUG()                                                  \
 do {                                                           \
        asm volatile("ud2");                                    \
-       for (;;) ;                                              \
 } while (0)
 #endif
 
_

reduces the size of i386 mm/vmalloc.o text by 56 bytes.

I wonder if there is any clever way in which we can do this without
introducing additional runtime cost.

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